I've noticed that Easter nearly always falls on the Sunday of Passover (and the second Sunday, if Passover happens to span 2 of them). According to various calendars on the web (google for "easter date" or "date of passover" without the quotes), the next several Easters which do not fall during Passover are 2005, 2008, 2016, and 2024. In fact, based on the calculations on http://quasar.as.utexas.edu/BillInfo/ReligiousCalendars.html, it happens exactly 3 out of every 19 years.
According to a lot of these sites, both the date of Easter and the Hebrew calendar are based on a 19 year cycle called the Metonic cycle. But I'm worried, because according to math I did based on the numbers at http://scienceworld.wolfram.com/astronomy/MetonicCycle.html, the Hebrew calendar gains a day roughly every 214 years. So Passover may seem late this year (and in fact will be a week later in 2 years), but in about 1500 years, we'll have our first Passover that starts in May! Or, if I got my sign wrong and the Hebrew calendar is actually losing time, then in roughly 3000 years, Passover will start in March about half the time, instead of rarely, and will occasionally start during the solar winter. Considering that Mt Sinai was theoretically about 3000 years ago, we've probably already gained (or lost?) about 2 weeks since then. Furthermore, whichever way the correct answer goes, we have only 39166 years before Passover always starts in the Northern Hemisphere's autumn, between roughly September 25th and October 25th. Yes, I am actually troubled by this.
If the Hebrew calendar is instead based on a Callipic or Hipparchic cycle (links from that scienceworld link above), then we will only gain or lose a day roughly every 325 years or 51472 years, respectively. (The problem with the Hipparchic cycle is that it seems to gradually lose the lunar cycle (by a day every 304 years), so after 4488 years, every Hebrew month will start on a full moon instead of a new moon, even though it will still be at roughly the right time in the solar calendar.)
Does anyone know anyone who knows more about this than what I do after surfing the web for 10 minutes? Because I'm not satisfied by my own calculations...